The present invention relates to a spray powder for producing wear resistant coatings on the bearing and friction faces of machine parts subjected to sliding friction, such as, in particular, piston rings in high rpm Diesel engines, the friction faces of synchronizing discs, or the fire lands of the bottoms of pistons in internal-combustion engines, with such coatings being preferably applied in a plasma spray process.
In practice, wear resistance is increased by providing the bearing faces of machine parts which are subjected to sliding friction primarily with electrochemically applied hard chromium coatings, or with thermally sprayed-on protective coatings, preferably of metals or metal alloys. Molybdenum, in particular, has been found to be excellently suitable as a spray material for coating primarily the bearing faces of piston rings used in internal-combustion engines. The molybdenum is here applied either over the entire surface area, or coatings are applied in a so-called sprayed-over form, or grooves and recesses are worked into the bearing faces which are then filled with the spray material and these are called coatings in unilaterally or bilaterally chambered form.
Molybdenum spray coatings, most of all, have excellent resistance to burn traces, while their wear resistance is inferior to that of electrochemically applied hard chromium coatings. Additionally, molybdenum spray coatings are relatively brittle so that, particularly under extreme loads, there exists the danger of the coatings breaking out (delaminating), this being the case with chambered as well as with sprayed-over rings.
For that reason, it has been attempted, particularly in order to improve the wear resistance of such coatings, to alloy or mix other elements to the molybdenum, and additionally to add hard substances to the spray powders. For example, the spray powders disclosed in DE-AS 2,433,814 include molybdenum and 0.5 to 45% iron, cobalt, nickel, titanium, vanadium, chromium, aluminum, tungsten, tantallum, rhenium and/or zirconium as well as 0.8 to 10% silicon and, according to DE-OS 2,841,552, spray powders are provided in which 3 to 40% aluminum alloy are mixed to the molybdenum. Both spray powders may possibly additionally contain hard substances based on carbides, nitrides, oxides and/or intermetallic compounds. These measures resulted primarily in an improvement of the wear resistance of such sprayed layers, but this improvement of wear resistance costs such coatings part of their resistance to burn traces or results in the coatings becoming more brittle so that under extreme loads, there are even more breakouts in the coatings.